MIGRATION AND DISTRIBUTION OF PLANTS 437 



were not too wide, and lodged in the fresh-water pools, or lakes, or 

 streams of another near-by continent. 



620. Present climatic pressures. Other climatic pressures 

 also existed and continue to the present time. In the humid 

 tropics large numbers of individuals of different species are prop- 

 agated, which produce a pressure northward and southward 

 from this point, but those moving southward on the northern 

 hemisphere come in contact with those moving northward, and 

 here a lateral pressure is exerted which crowds the plants to 

 the west and east. Pressures also exist in the borders of arid 

 regions. The fertility of aggressive species wherever they occur 

 tends to produce pressures in all directions. 



II. BARRIERS TO PLANT MIGRATION. 



621. There are a number of barriers which plants meet in 

 their migration over the surface of the earth. In general terms 

 we might speak of four when looking at the world as a whole. 

 First. Kinds of climate. Regions of great heat or cold, of dry- 

 ness or moisture, etc. All these regions oppose obstacles to the 

 entrance and passage of plants which are accustomed to live in 

 different climates. Second. Kinds of soil. For example, the 

 alkaline deserts and the great salt steppes present effectual bar- 

 riers to the passage of plants not provided with adaptations 

 which would enable them to live under such extreme conditions. 

 Third. Discontinuity of land. Here bodies of water present a 

 barrier to the passage of plants from one continent to another, 

 or from one island to another, which are separated by broad 

 lakes or seas. A good illustration of this is shown in the relation 

 of the continents of the southern hemisphere as compared with 

 those of the northern hemisphere already pointed out. Fourth. 

 Mountain chains. High mountain chains, because of the great 

 cold, often form impassable barriers for plants. Good illustrations 

 of this are shown in a comparison of the number of species of 

 plants in Europe and North America and their distribution. 

 Under the high climatic pressures which existed, for example, in 

 glacial times, the plants of North America met with no barrier 



